1. Field of the Invention
The present invention pertains to a linear frequency discriminator which may include a reference stabilized center frequency. Discriminators of this type are useful in video tape recorders and are especially useful in the SECAM television system where the color information is transmitted in the form of color difference signals using a frequency modulated subcarrier. In order to provide a color image free of color errors and random drift in the reference background color it is necessary that the color signal frequency discriminator produce from the frequency modulated color difference signals, video color difference signals in which the amplitude faithfully corresponds to the frequency variations representing color difference information. This is necessary to insure that the color difference signals when compared with the luminance signal will produce color signals at the color display device corresponding to the color signals contained in the frequency modulated subcarrier. Since the quiescent discriminator output (no frequency modulation of the subcarrier) provides for an essentially black and white picture which is tinted to form the current color picture when frequency modulation of the color subcarrier occurs, it is also necessary to insure that an unmodulated color subcarrier signal produces either zero output or the correct reference level when no frequency modulation is present. If the discriminator drifts or has been mistuned the background color will drift or be in error. Accordingly, the system requires a linear frequency discriminator for the frequency modulated color subcarrier information and one which not only is carefully adjusted but free of tuning drift.
2. Description of the Prior Art
Prior art systems have used frequency discriminators of conventional design such as the Travis or Foster Seely types which convert the frequency modulated signal to a pair of signals one of which increases in amplitude as the frequency of the incoming signal is increased and one of which decreases in amplitude as the frequency increases and vice versa. By taking the amplitude difference of these signals a base band signal can be derived the amplitude of which is a measure of the instantaneous frequency of the incoming carrier signal. These systems require complex networks to derive the signals containing the frequency difference information. It is difficult to derive networks which maintain a linear frequency amplitude conversion characteristic over a wide range of frequencies.
A second method makes use of the product between the incoming signal and a phase shifted complement of the incoming signal such that the phase difference between the two signals has a linear relationship to the frequency modulation of the carrier. Detectors of this type include the quadrature detector and the phase locked loop, both of which exhibit a sinusoidal transfer characteristic which is linear for a narrow range of frequency modulation only.